1. Field of the Invention
The present invention relates to a fixing device which effects fixing of a toner image on an image forming medium by means of heat rollers in an image forming apparatus.
2. Description of the Prior Art
In general, a means as shown in FIG. 1 has been previously used for fixing toner images formed on various sizes of paper sheets in an image forming apparatus.
In this apparatus, a heat roller 1 includes heater lamps 3 and 4 mounted inside a hollow roller 2. Heater lamp 3 has a heating coil element 3a whose length corresponds to the width of an A4 paper sheet 7a, and heater lamp 4 has a heating coil element 4a whose length corresponds to the width of an A3 paper sheet 7b. Each of heater lamps 3 and 4 is selectively heated to around 200.degree. C. and a supporting roller 5 is pressed against heat roller 1. Thermal fixing of toner image 6 is effected on A4 sized paper sheet 7a by passing the sheet 7a between heat roller 1 and supporting roller 5 while heater lamp 3 is heated. For thermal fixing of a toner image on A3 sized paper sheet 7b, heater lamp 4 is heated. Thus, heated region control is effected by using either heater lamp 3 or 4 with different heating widths corresponding to the widths of paper sheets.
However, fixing devices such as this with built-in heater lamps are somewhat bulky, since the roller diameter must be enlarged in order to prevent contact of the heater lamps with each other.